Protostars and Planets VI, Heidelberg, July 15-20, 2013
Cosmic-ray ionization of a molecular cloud interacting with the supernova remnant W28
Vaupré, Solenn (Institut de Planétologie et d\'Astrophysique de Grenoble)
Ceccarelli, Cecilia (Institut de Planétologie et d\'Astrophysique de Grenoble)
Hily-Blant, Pierre (Institut de Planétologie et d\'Astrophysique de Grenoble)
Dubus, Guillaume (Institut de Planétologie et d\'Astrophysique de Grenoble)
Montmerle, Thierry (Institut d\'Astrophysique de Paris)
Molecular clouds irradiated by cosmic-rays (CR) experience dramatic modifications in their physical conditions, dynamical evolution and chemical composition. Two main effects of the interaction can be observed with ground-base telescopes: CR lead to (1) gamma-ray emission due to the interaction of very-high energy CR (> GeV) with the dense gas and (2) ionization of the gas deep inside the cloud by low-energy CR (< GeV). CR secondary photons are the dominant source of ionization in UV-shielded dense gas. This ionization affects the star formation process, and triggers an ion-neutral chemistry which eventually leads to the formation of complex molecules.
With the IRAM 30m radiotelescope, we observed a molecular cloud close to the Supernova Remnant W28 and overlapping with bright TeV emission. On the one hand, CO isotopologues were used to determine the physical conditions under the Large Velocity Gradient assumption. On the other hand, HCO+ and DCO+ were used as tracers of the ionization degree. Altogether, the abundances and abundance ratios of these gas-phase species are compared to theoretical predictions of the chemical structure of UV shielded clouds, by means of the CR Dominated Region (CRDR) code developed at IPAG and presented in this work too. This comparison allows us to constrain the CR ionization rate in the clouds close to W28.
We found that the cosmic-ray ionization rate ζ is at least 30 times larger than the standard value in dense clouds ζ ∼ 10^17 s−1. This is one of the very few clouds ever observed so far with such an enhanced value (Indriolo et al. 2010, Ceccarelli et al. 2011). Besides, a bistable behavior of the chemical abundances is observed, where a High- and a Low- Ionization Phases coexist (HIP/LIP) (Le Bourlot et al. 1993).
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